1. Field of the Invention
The present invention relates to a semiconductor light emitting device such as a light emitting diode, a semiconductor laser device, or the like. More particularly, the present invention relates to a semiconductor light emitting device in which a light emitting section made of an AlGaInP type semiconductor material is formed on a GaAs substrate.
2. Description of the Related Art
A semiconductor device using an AlGaInP type semiconductor material has been used as a visible range light emitting device for its advantages such as the capability of achieving the lattice match between a GaAs substrate and the AlGaInP type semiconductor material and that it has the greatest direct transition band gap among the group III-V compound semiconductor materials. Known semiconductor devices using such a material include light emitting diodes, semiconductor laser devices, and the like.
Where an AlGaInP type semiconductor layer is epitaxially grown on a GaAs substrate by using an MOCVD (metal-organic chemical vapor deposition) method, or an MBE (molecular beam epitaxial) method, it is necessary in order to achieve a desirable crystallinity that an impurity such as oxygen is not introduced and that a desirable two-dimensional growth is achieved.
In view of this, Japanese Laid-Open Publication No. 6-57149 discloses method for producing a conventional AlGaInP type semiconductor laser device as shown in FIG. 9 in which an AlGaInP layer is grown on a plane of an n-GaAS substrate 01 whose principal plane is inclined from the (100) plane toward the [011] orientation.
In this method, first, an n-GaInP or n-AlGaInP buffer layer 82, an n-AlGaInP cladding layer 83, a GaInP active layer 84, a p-AlGaInP cladding layer 85 and a p-GaAs cap layer 86 are grown by an MOCVD method in this order on the n-Gas substrate 81 whose principal plane is inclined from the (100) plane toward the [011] orientation. Then, an SiO2 film 87 is formed on the p-GaAs cap layer 86, and the central portion of the SiO2 film 87 is etched in a stripe pattern. An electrode 811 is formed over the SiO2 film 87, and an electrode 810 is formed on the reverse side of the n-GaAs substrate 81.
In this conventional method for producing a semiconductor laser device, a GaInP or AlGaInP buffer layer is grown on the principal plane of a GaAs substrate which is inclined from the (100) plane toward the [011] orientation, and then an AlGaInP type light emitting section is grown by an MOCVD method.
In this method, however, the effects of inclining the principal plane of the GaAs substrate from the (100) plane toward the [011] orientation do not sufficiently manifest themselves for the following reasons. First, the GaInP or AlGaInP buffer layer is grown directly on the GaAs substrate. Second, a single composition is used for the buffer layer.
According to one aspect of this invention, there is provided a semiconductor light emitting device at least including: a GaAs substrate whose principal plane is inclined from a (100) plane in a [011] orientation; a first buffer layer of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61) provided on the principal plane of the GaAs substrate; a second buffer layer of AlyGazIn1xe2x88x92yxe2x88x92sP (0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61) provided on the first buffer layer; a first cladding layer of AlsGatIn1xe2x88x92axe2x88x92tP (0xe2x89xa6sxe2x89xa61 and 0xe2x89xa6txe2x89xa61) provided on the second buffer layer; an active layer provided on the first cladding layer: and a second cladding layer provided on the active layer, wherein an Al content a of the first cladding layer is larger than an Al content y of the second buffer layer.
In one embodiment of the invention, the principal plane of the Gays substrate is inclined from the (100) plane toward the [011] orientation by an angle equal to or greater than about 2xc2x0.
In one embodiment of the invention, the Al content y of the second buffer layer is equal to or greater than about 0.3 and less than or equal to about 0.8.
In one embodiment of the invention, a growth temperature for the second buffer layer is different from that for the first cladding layer.
In one embodiment of the invention, a growth temperature for the second buffer layer is equal to that for the first cladding layer.
In one embodiment of the invention, a growth temperature for the first buffer layer is equal to that for the second buffer layer.
In one embodiment of the invention, the second buffer layer is grown while changing a growth temperature therefor in a stepwise or continuous manner.
In one embodiment of the invention, the first cladding layer is grown while changing a growth temperature therefor in a stepwise or continuous manner.
In one embodiment of the invention, the semiconductor light emitting device further includes a current diffusing layer on the second cladding layer.
In one embodiment of the invention, the semiconductor light emitting device further includes a current blocking layer between the second cladding layer and the current diffusing layer.
In one embodiment of the invention, the current blocking layer is provided in a central portion of the semiconductor light emitting device.
In one embodiment of the invention, the current blocking layer is provided in a peripheral portion of the semiconductor light emitting device.
In one embodiment of the invention, the active layer is a quantum well active layer obtained by depositing a number of quantum well layers and a number of barrier layers in an alternating pattern.
In one embodiment of the invention, the semiconductor light emitting device further includes a is current blocking layer provided on the second cladding layer and a cap layer provided on the current blocking layer.
In one embodiment of the invention, the semiconductor light emitting device further includes a light reflecting layer provided closer to the GaAs substrate with respect to the first cladding layer.
According to another aspect of this invention, there is provided a method for producing a semiconductor light emitting device through a vapor phase deposition method on a principal plane of a GaAs substrate which is inclined from a (100) plane toward a [011] orientation, the method including the steps of: (a) growing a first buffer layer of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61) on the principal plane of the GaAs substrate; (b) growing a second buffer layer of AlyGaxIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6zxe2x89xa61) on the first buffer layer; and (c) sequentially growing a first cladding layer of AlzGatIn1xe2x88x92sxe2x88x92tP (0xe2x89xa6sxe2x89xa61 and 0xe2x89xa6txe2x89xa61) on the second buffer layer, an active layer on the first cladding layer, and a second cladding layer on the active layer, wherein an Al content a of the first cladding layer is larger than an Al content y of the second buffer layer.
In one embodiment of the invention, the step (a) is performed at a growth temperature between about 600xc2x0 C. and about 700xc2x0 C.; the step (b) is performed while increasing a growth temperature from a temperature between about 600xc2x0 C. and about 700xc2x0 C. to a temperature between about 700xc2x0 C. and about 850xc2x0 C.; and the step (c) is performed at a growth temperature between about 700xc2x0 C. to about 850xc2x0 C.
In one embodiment of the invention, the vapor phase deposition method is an MOCVD method.
In one embodiment of the invention, the vapor phase deposition method is an MBE method.
In one embodiment of the invention, the principal plane of the GaAs substrate is inclined from the (100) plane toward the [011] orientation by an angle equal to or greater than about 2xc2x0.
In one embodiment of the invention, the Al content y of the second buffer layer is equal to or greater than about 0.3 and less than or equal to about 0.8.
In one embodiment of the invention, a growth temperature for the step (b) is different from that for the step (c).
In one embodiment of the invention, a growth temperature for the step (b) is equal to that for the first cladding layer in the step (c).
In one embodiment of the invention, a growth temperature for the step (a) is equal to that for the step (b).
In one embodiment of the invention, the step (b) is performed while changing a growth temperature therefor in a stepwise or continuous manner.
In one embodiment of the invention, the first cladding layer is grown in the step (c) while changing a growth temperature therefor in a stepwise or continuous manner.
In one embodiment of the invention, a current diffusing layer is provided on the second cladding layer.
In one embodiment of the invention, a current blocking layer is provided between the second cladding layer and the current diffusing layer.
In one embodiment of the invention, the current blocking layer is provided in a central portion of the semiconductor light emitting device.
In one embodiment of the invention, the current blocking layer is provided in a peripheral portion of the semiconductor light emitting device.
In one embodiment of the invention, the active layer is a quantum well active layer obtained by depositing a number of quantum well layers and a number of barrier layers in an alternating pattern.
In one embodiment of the invention, a current blocking layer is provided on the second cladding layer and a cap layer is provided on the current blocking layer.
In one embodiment of the invention, a light reflecting layer is provided closer to the GaAs substrate with respect to the first cladding layer.
In one embodiment of the invention, a growth temperature is increased after the step (b), and the step (c) is performed thereafter.
The functions of the present invention will now be described.
According to the present invention, the principal plane of the GaAs substrate is inclined from the (100) plane toward the [011] orientation. As a result, it is less likely that oxygen is introduced into the AlGaInP type semiconductor layer to be grown thereon, thereby obtaining a desirable crystallinity. In order for this effect to manifest itself sufficiently, it is preferred that the principal plane is inclined by an angle of equal to or greater than about 2xc2x0. On such an inclined principal plane, the first buffer layer of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa61) is formed. As a result, the diffusion of an impurity from the substrate into the light emitting section is suppressed, and the planarity of the surface of the substrate is improved, i.e., the unevenness is reduced. Of course, the inclined plane orientation is conserved. Unlike the conventional buffer layer of GaInP or AlGaInP, the GaAs or AlGaAs first buffer layer of the present invention has the same composition as that of the substrate. As a result, unevenness is not introduced at the junction therebetween, and it is possible to grow thereon a buffer layer having a smaller number of defects and thus a desirable crystallinity with the inclined plane orientation being kept unchanged. Moreover, the AlGaInP first cladding layer is provided on the AlGaAs (or GAs) first buffer layer via the AlGaInP (or GaInP) second buffer layer. As a result, the crystallinity of the light emitting section, particularly the active layer, is significantly improved, thereby improving the emission efficiency. Alternatively, an AlGaInP (or GaInP) buffer layer structure including two or more layers therein may be provided on the AlGaAs (or GaAs) first buffer layer.
According to the present invention, the Al content s of the first cladding layer is larger than the Al content y of the second buffer layer. As a result, it is possible to obtain a semiconductor light emitting device in which the crystallinity of the light emitting section, particularly the active layer, is significantly improved, thereby improving the emission efficiency. For example, when a cladding layer having a large Al content is grown directly on a GaAs (or AlGaAs) buffer layer, a desirable crystallinity is not obtained. However, as in the present invention, when the second buffer layer having a small Al content is first grown on the GaAs (or AlGaAs) buffer layer, and then the first cladding layer having a large Al content is grown on the second buffer layer, an improved crystallinity is obtained. Alternatively, the composition of the second buffer layer and/or the first cladding layer may be changed gradually. The change of the composition may be either stepwise or continuous.
The Al content y of the second buffer layer is preferably equal to or greater than about 0.3 and less than or equal to about 0.8. In such a case, it is possible to obtain a semiconductor light emitting device in which the crystallinity of the light emitting section, particularly the active layer, is significantly improved, thereby improving the emission efficiency.
The growth temperature for the second buffer layer may be different from that for the first cladding layer. For example, a cladding layer having a large Al content (the first cladding layer) may be grown at a higher growth temperature while a buffer layer having a small Al content (the second buffer layer) may be grown at a lower growth temperature, because the optimal growth temperature for an AlGaInP layer having a large Al content is relatively high.
The growth temperature for the second buffer layer may be changed gradually (in a stepwise or continuous manner). For example, the second buffer layer may be grown while increasing the growth temperature during the growth process from the optimal growth temperature for the GaAs (or AlGaAs) first buffer layer to the optimal growth temperature for the AlGaInP (or GaInP) second buffer layer. In such a case, the crystallinity can be further improved. Similarly, the growth temperature for the first cladding layer may be changed gradually.
A current diffusing layer may be provided on the second cladding layer. In such case, the injected current flow can be diffused through the current diffusing layer so as to efficiently utilize the entire active layer for emitting light, thereby significantly improving the emission efficiency.
A current blocking layer may be provided between the second cladding layer and the current diffusing layer. In such a case, the injected current flow can be even better diffused through the current diffusing layer so as to more efficiently utilize the entire active layer for emitting light, thereby further improving the emission efficiency.
The current blocking layer may be provided in the central portion of the device. In such a case, it is possible to diffuse the injected current flow toward the peripheral portion of the device and to efficiently extract the generated light from the device. Alternatively, the current blocking layer may be provided in a peripheral portion of the device. In such a case, it is possible to localize the current flow to the central portion of the device, thereby increasing the current density, and to efficiently extract the generated light from the device.
A light reflecting layer may be provided closer to the GaAs substrate with respect to the first cladding layer. In such a case, a portion of the generated light, which would otherwise be absorbed by the substrate, can be reflected by the light reflecting layer so that it can be extracted from the device, thereby significantly improving the efficiency of use of the generated light.
The present invention can be used to provide a semiconductor laser device by providing a current blocking layer and a cap layer on the second cladding layer.
Thus, the invention described herein makes possible the advantage of providing a semiconductor light emitting device including a GaAs substrate whose principal plane is inclined from the (100) plane toward the [011] orientation, on which an AlGaInP type semiconductor layer with a desirable crystallinity is grown, thereby significantly improving the emission efficiency.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.